Field of the Invention
This invention relates to an electric power supply device and an image forming apparatus having the electric power supply device. More specifically, this invention relates to an electric power supply device which can reduce the amount of power consumption and an image forming apparatus having such an electric power supply device.
Description of the Related Art
An image forming apparatus with electrophotographic technology may be a MFP (Multi Function Peripheral) having a scanner function, a facsimile function, a copying function, a function as a printer, a data transmitting function and a server function, a facsimile device, a copying machine, a printer or the like.
FIG. 8 schematically shows a block diagram of a circuit structure of an electric power supply circuit board and control circuit board, when a conventional image forming apparatus is in the normal condition.
Referring to FIG. 8, the conventional image forming apparatus is equipped with an electric power supply circuit board 1001, an electric wire resistor (a wire harness) R105, and a control circuit board 1200 which controls behavior of each element of the image forming apparatus. Electric power supply circuit board 1001 and control circuit board 1200 are electrically connected via electric wire resistor R105. Electric power supply circuit board 1001 provides direct current power to control circuit board 1200 via electric wire resistor R105.
Electric power supply circuit board 1001 includes a transformer T101, a secondary side rectification and smoothing circuit (a condenser) SM101, a constant voltage control unit 1010, and a transistor TR101. Transformer T101 is a DC-DC converter. Transistor TR101 is a P channel type MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor).
A primary side coil of transformer T101 is connected with the source and the drain of transistor TR101. A secondary side coil of transformer T101 is connected with secondary side rectification smoothing circuit SM101. The one output terminal of secondary side rectification smoothing circuit SM101 is connected with the one end of electric wire resistor R105. The other output terminal of secondary side rectification smoothing circuit SM101 is connected with ground potential. A contact point P110 is located on a conducting wire connecting the one output terminal of secondary side rectification smoothing circuit SM101 and electric wire resistor R105. Constant voltage control unit 1010 adopts contact point P110 as the remote sensing location, and controls outputting electrical voltage of electric power supply circuit board 1001 at constant voltage, to keep electrical potential of contact point P110 constant.
Control circuit board 1200 includes resistor elements R111, R112, R113, R114, and R115, switches SW111, SW112, SW113, and SW114, and functional circuits CR101, CR102, CR103, CR104, and CR105. Each of resistor elements R111, R112, R113, R114, and R115 is a pattern resistor. Functional circuit CR101 is a HDD (Hard Disk Drive) driver, and for example, performs guaranteed behavior at electrical voltage of 4.75V (voltage) to 5.25V. Functional circuit CR102 is a USB (Universal Serial Bus) driver, and for example, performs guaranteed behavior at electrical voltage of 4.85V to 5.15V. Functional circuit CR103 is an electrical power saving circuit, and is enabled at all times. Functional circuit CR104 is an image forming circuit which controls image forming. Functional circuit CR105 is a facsimile circuit which controls transmitting and receiving of facsimile.
The one end of each of resistor elements R111, R112, R113, R114, and R115 is connected with electric wire resistor R105. Resistor element R111, switch SW111, and functional circuit CR101 are connected in series in this order. Resistor element R112, switch SW112, and functional circuit CR102 are connected in series in this order. Resistor element R113 and functional circuit CR103 are connected in series in this order. Resistor element R114, switch SW113, and functional circuit CR104 are connected in series in this order. Resistor element R115, switch SW114, and functional circuit CR105 are connected in series in this order. Functional circuit CR103 controls on-off of each of switches SW111, SW112, SW113, and SW114, by transmitting each of electric power supply signals REM101, REM102, REM103, and REM104, based on the behavior of the image forming apparatus.
Specifications of inputting electrical voltage are set for control circuit board 1200. In case that electrical voltage of the contact point of the input of each of functional circuits CR101, CR102, CR103, CR104, and CR105 is less than the inputting electrical voltage of the specifications of control circuit board 1200, a problem may be caused with the functional circuits. Then, considering the electrical voltage drop caused by electric wire resistor R105, and resistor elements R111, R112, R113, R114, and R115, the outputting electrical voltage of electric power supply circuit board 1001 is controlled, to keep electrical voltage of each of contact points P101, P102, P103, P104, and P105 of the input of each of functional circuits CR101, CR102, CR103, CR104, and CR105 higher than the inputting electrical voltage of the specifications of control circuit board 1200 at all times.
For example, in case that the inputting electrical voltage of the specifications of control circuit board 1200 is 5.0V, the electrical voltage of contact point P110 should be controlled at 5.1V at all times, by constant voltage control.
It is assumed that, in case that the electrical voltage of the contact point P110 is kept at 5.1V at all times, by constant voltage control, switches SW111, SW112, SW113, and SW114 are turned on (in case that the total amount of electrical current flows in control circuit board 1200 is maximum). In the event, electrical current flows in each of functional circuits CR101, CR102, CR103, CR104, and CR105, and electrical voltage at electric wire resistor R105 and each of resistor elements R111, R112, R113, R114, and R115 decreases significantly. In consequence, the electrical voltage of contact point P101 becomes at 5.02V, the electrical voltage of contact point P102 becomes at 5.04V, the electrical voltage of contact point P103 becomes at 5.07V, the electrical voltage of contact point P104 becomes at 5.01V, the electrical voltage of contact point P105 becomes at 5.03V, and the electrical voltage of contact point P106 becomes at 5.07V. Herewith, the circumstances in which the electrical voltage of the contact point of the input of each of functional circuits CR101, CR102, CR103, CR104, and CR105 becomes less than the inputting electrical voltage of the specifications of control circuit board 1200 can be avoided.
The patent document 1 below discloses an electric power supply device which provides a plurality of load boards with common electric power supply voltage. The device comprises a plurality of remote detecting means which detect electrical voltage of the inputting portion of a plurality of load devices, an outputting means which determines outputting electrical voltage of the electric power supply device by a plurality of detected values acquired by the remote detecting means and outputs the same, and a constant voltage control means which controls the outputting means by constant voltage control.    [Document 1] Japan Patent Publication No 2003-169470
FIG. 9 schematically shows a block diagram of a circuit structure of an electric power supply circuit board and a control circuit board, when a conventional image forming apparatus is in the electrical power saving state.
Referring to FIG. 9, in case that the image forming apparatus is in the electrical power saving state (an energy saving mode), all of the switches SW111, SW112, SW113, and SW114 are turned off. In this instance, electrical current flows only in functional circuit CR103, out of functional circuits in control circuit board 1200. Because the total amount of electrical current flows in control circuit board 1200 lowers, the electrical voltage drop at electric wire resistor R105 and resistor element R113 is almost exactly zero. In consequence, electrical voltage of contact point P103 becomes at 5.1V which is higher than necessary, although 5.0V is sufficient as electrical voltage of contact point P103, which is the inputting electrical voltage of the specifications of control circuit board 1200. Hence, the amount of power consumption increases.
Specifications of energy saving, needed for electronic devices like image forming apparatuses, become all the more severe recently. To keep pace with such the strict specifications of energy saving, it is important to reduce the amount of power consumption.